1. Field of the Invention
The present invention relates to a turbomolecular vacuum pump particularly adapted for use with a semiconductor manufacturing apparatus for generating high vacuum.
2. Description of the Prior Art
A conventional turbomolecular vacuum pump has hitherto been known which is described in the April-June 1983 edition of "Journal of Vacuum Science Technology" in the article entitled "A new type of turbomolecular vacuum pump bearing", on pages 224 to 227. Such a pump is illustrated in FIG. 5 in a vertical cross section. In FIG, 5, the turbomolecular vacuum pump illustrated includes a plurality of rotor vanes 1 integrally formed with the radially outer periphery of a rotary member 1a in an axially spaced parallel relation with each other, and a plurality of stator vanes 2 housed in a vertically disposed cylindrical housing 3 and fixedly mounted on the inner surface of the housing 3, the stator vanes 2 being disposed in a spaced parallel relation with each other and interlaced with the annular rotor vanes 1 in a face-to-face relation with each other. The rotary member 1a is vertically disposed in the cylindrical housing 3 and integrally formed with a rotary shaft 15 which is rotatably supported at its upper end through an upper bearing 6a in the form of a rolling element bearing such as a ball bearing by a cylindrical frame 10 which is fixedly mounted at its lower end on a base plate or end wall 9 integrally formed with or otherwise fixedly connected with the cylindrical housing 3 at its lower end. The rotary shaft 15 is also supported at its lower end by the base plate 9 through a bearing 6b in the form of a rolling element bearing such as a ball bearing. Disposed inside the cylindrical frame 10 is an electric motor 4 for rotating the rotary shaft 15 together with the rotary member 1a, the electric motor 4 including a stator 4a fixedly mounted on the inner peripheral surface of the cylindrical frame 10, and a rotor 4b disposed radially inwardly of the stator 4a and fixedly mounted such as by shrink fitting on the outer peripheral surface of the rotary shaft 15. The base plate 9 has an outlet port 9b formed therethrough to which a discharge pipe 7 is connected for discharging a fluid such as, for example, air in the housing 3 to the outside. The cylindrical housing 3 is integrally formed at its open end (the upper end in FIG. 5) with an annular flange 16 for mounting the turbomolecular pump to an apparatus requiring high vacuum (not shown) such as, for example, a semiconductor manufacturing apparatus. Here, it is to be noted that the rotor vanes 1 and the stator vanes 2 are appropriately configured such that during rotation of the rotor vanes 1, a unidirectional flow of air within the cylindrical housing 3 is created in a direction from an inlet opening or port 3a in the upper end of the housing 3 toward the outlet port 9b.
In operation, when the turbomolecular vacuum pump as contructed in the above manner and mounted at the annular flange 16 to an apparatus (not shown) requiring high-vacuum is driven to run by means of the electric motor 4, that is when the rotary member 1a, integrally formed with the rotary shaft 15, is driven to rotate at speeds of usually several ten thousand rpm's by energizing the electric motor 4, the fluid contained in the apparatus requiring high-vacuum (not shown) is caused to discharge to the outside by way of the inlet opening 3a in the flanged end of the housing 3, the clearances formed between the rotor vanes 1 of the rotary member 1a and the stator vanes 2 on the inner surface of the cylindrical housing 3, the annular chamber 3b defined in the housing 3 between the base plate 9 and the adjacent end surface of the rotary member 1a, the outlet port 9b in the base plate 9 and the discharge pipe 7, as indicated by broken lines 11a and 11b in FIG. 5, so that the pressure inside the high-vacuum requiring apparatus (not shown) is gradually reduced to create a high vacuum therein. In this connection, it is to be noted that the pressure in the turbomolecular vacuum pump gradually decreases from the outlet port 9b side near the atmosphere toward the inlet opening 3a side near the high-vacuum requiring apparatus.
Usually, the greater the number of rotor vanes 1, the higher the vacuum developed in the high-vacuum requiring apparatus will be. Also, as the rpm's of the rotor vanes 1 increase, the performance of the pump is enhanced so that the size and the weight thereof can be reduced accordingly.
In the above-described conventional turbomolecular vacuum pump, however, rolling element bearings such as ball bearings are employed for the bearings 6a and 6b so that there arise various problems such as a shortened service life due to high-speed rotation of the rotary shaft 15 and hence of the rotary member 1a, mechanical power loss resulting from bearing functional losses, reduced rotational performance, low assemblability and the like.
In order to cope with the above-mentioned various problems, the present inventors proposed an improved turbomolecular vacuum pump which is described in a Japanese Utilitiy Model Application under Ser. No. 60-78395 (78395/1985). Such an improved pump is illustrated in FIG. 6. In this pump, a rotary shaft 15 is supported at its upper end through a touch-down bearing 14 by a cylindrical frame 10 fixedly mounted on a base plate 9, and at its lower end by the base plate 9 through a spherical-type spirally grooved bearing 12 which is disposed in an inwardly or upwardly facing recess 9a formed in the base plate 9, the recess 9a being filled with a lubricating oil such as one usually having a very low saturated vapour pressure which is, for example, below 10.sup.-11 Torr at normal temperatures. The construction and arrangement of this pump are substantially the same as those of the pump illustrated in FIG. 5.
Now, referring to the rotational performance of the improved turbomolecular vacuum pump, the rotary shaft 15 mounting thereon the rotary member 1a is rotatably supported at its upper and lower ends by means of the touch-down bearing 14 and the spherical-type spirally grooved bearing 12, respectively. When the rotary shaft 15 is driven to rotate by means of the electric motor 4, it is supported by both bearings 12 and 14 at relatively low rpm ranges, but as the rpm's of the rotary shaft 15 increase to a level above several thousand rpm's, the upper end of the rotary shaft 15 is automatically released from or drawn out of engagement with the upper touch-down bearing 14 by the gyroscopic action of the rotary member 1a mounted on the upper end of the rotary shaft 15 so that it begins to rotate without contacting the touch-down bearing 14.
On the other hand, the lower end of the rotary shaft 15 is in metal-to-metal contact with the lower bearing 12 when the rotary shaft 15 is not rotated, but it comes into sliding contact with the lower bearing 12 through the intermediary of a film of lubricating oil 13 drawn into the grooves on the lower spherical spiral bearing 12 during rotation of the rotary shaft 15. Thus, when rotating, the rotary shaft 15 is supported by a fluid film provided by the lubricating oil 13.
In the past, an induction motor was generally employed as the electric motor 4 for driving the rotary member 1a, and such an induction motor includes a stator 4a and a rotor 4b disposed radially in a concentric relation with each other. The stator 4a and the rotor 4b are disposed inside a cylindrical support frame 10, and hence the radial dimensions of such a stator and rotor are limited by the frame 10 so that the induction motor 4 must have a relatively large axial length to produce a certain level of output power as required. As a result, the distance between the upper and lower bearings 14 and 12 is increased, thus making the center of gravity of the rotary member 1a higher or farther away from the lower bearing 12 and destabilizing the high-speed rotation of the rotary member 1a. Also, it is difficult to reduce the axial size of the motor 4 and hence of the entire pump. Moreover, the rotary member 4b was shrink fitted onto the rotary shaft 15 so that the rotor 4b, being subject to centrifugal forces which increase in accordance with increases in rpm's of the motor 4, is forced to expand radially under the action of those centrifugal forces. As a result, the mechanical connection of the rotor 4b with the rotary shaft 15 became less secure and insufficient during the high speed operation of such an induction motor. Accordingly, there were restraints on high-speed operation of turbomolecular pumps. In addition, due to the fact that the cylindrical frame 10 is disposed radially outwardly of the rotary shaft 15 and fixedly mounted on the base plate 9 with the motor stator 4a secured to the inner peripheral surface thereof, the construction and assembly of the entire device was relatively complicated and inefficient.